Method of treating chemical cellulose pulp

ABSTRACT

A method of bleaching chemical cellulose pulp from an alkaline (e.g. kraft) pulping process in a chlorine dioxide stage includes (without between step washing) bleaching the pulp in a first chlorine dioxide step, and adjusting the pH of the pulp in the first chlorine dioxide step so that the final pH of the step is over 4; and then (b) effecting acid treatment of the chemical cellulose pulp at a pH of between 2-5 (preferably 2.5-4) and a temperature of over 80° C. (preferably between about 90-110° C.). The temperature in the first chlorine dioxide stage is preferably over 75° C., e.g. between about 80-100° C., and for a time of less than ten minutes, with a chlorine dioxide dosage of between about 0.5-1.5% active chlorine. Preferably a second chlorine dioxide step is practiced after the acid treatment, preferably at substantially the same conditions as the first chlorine dioxide step. The method minimizes the consumption of chlorine dioxide to bleach to a particular kappa number, minimizes the amount of equipment, enhances energy economy of the chlorine dioxide bleaching process, and minimizes the discharge of malodorous gases since they are oxidized by the first chlorine dioxide step.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] The invention relates to a method of treating chemical cellulosepulp from an alkaline pulping process in a chlorine dioxide stage whichincludes an acid treatment at a temperature of over 80° C.

[0002] Pulp mills have recently attempted to avoid the use of elementarychlorine, and also minimize the use of chlorine dioxide, forenvironmental protection and marketing reasons. Disadvantages caused byelementary chlorine include both noticeable malodorous gaseous emissionsand liquid effluents from chemical pulp mills into water systems. Liquidchlorine dioxide does not have odor problems on as large a scale aselemental chlorine, but it adversely affects water systems. Whencomparing these chlorine chemicals with each other by means of the AOXnumber designating the loading they incur on water systems, it is seenthat elementary chlorine is many times more detrimental than chlorinedioxide. The AOX number of chlorine is of the order of 4-7 and that ofchlorine dioxide is about 1-1.5, or even lower.

[0003] Therefore, sequences utilizing chlorine dioxide are still popularand environmentally feasible. There are many reasons for this. The priceof chlorine dioxide is very competitive compared with that of otherbleaching and delignifying chemicals. Also the strength and brightnessof pulp obtained by dioxide bleaching are good, in fact at leastapproximately of the same order as those obtained by using peroxide atthe same chemical consumption level (kg/admt).

[0004] Chlorine dioxide bleaching (D) is a well-known and widely usedbleaching procedure. Conventional chlorine dioxide bleaching is usuallyperformed at a temperature of 50-80° C. and a with a chlorine dioxidedosage of about 10-30 kg per ton of pulp. Typical sequences used are,for example, D_(o)ED₁ED₂ and OD_(o)ED₁ED₂ and modifications thereof. Thetreatment time in a D_(o) stage is usually shorter than in other Dstages, being for example, between about 30-90 minutes. The pH of thepulp decreases to the range of 1-3 towards the end of the D_(o) stage.In the D₁ and D₂ stages the treatment time is 2-3 hours and the pH alittle higher than in the Do stage. The chlorine dioxide stage (alltypes) is usually performed at a temperature of about 70° C., while thetreatment time in the D_(o) stage is 0.5-2 hours and in the D₁ and D₂stages 2-3 hours. Higher temperatures than about 70° C. have typicallybeen avoided as the low final pH in the range of 1-3 in the D stagecombined with a high temperature and long treatment time deterioratesthe strength properties of the pulp fibers.

[0005] Other elemental-chlorine-free bleaching processes, such as thoseusing peroxide and ozone, have also been introduced. As pulp bleachedwith chlorine dioxide has good properties, the development of ClO₂bleaching has been continued in conjunction with other elementalchlorine-free processes. One of the most important objects is to reducethe amount of chlorine dioxide needed in bleaching and thus to make ClO₂bleaching more environmentally acceptable.

[0006] Finnish patent application no. 944808 and WO publication no.96/12063 disclose an acid treatment (A), also known as a hot acid step,method for pulp by which the consumption of chlorine dioxide can bereduced. This treatment removes hexenuronic acids, which can react withchlorine dioxide and thus increase chemical consumption, from the pulp.The acids may be removed by adjusting the pH of the pulp to the range of2-5, preferably between 2.5-4, and treating at a temperature of over 80°C., preferably between 90-110° C., at a time between 30-300 minutes.During acid treatment the kappa number of the pulp is typically reducedby 2-9 units. When the sequence O-A-D-E-D is used, the consumption ofchlorine dioxide in the bleaching of hard wood pulp has reduced by30-40% at a brightness of ISO 88%. The corresponding consumption withsoft wood pulp has reduced by 10-20%. In both cases, the yield remainsnearly unchanged compared with bleaching without an acid (A) stage.

[0007] The present invention provides an alternative method of combininga hot acid step with a bleaching sequence utilizing chlorine dioxide.The invention provides a method which may be practiced in an economicalmanner, particularly in bleaching plants of existing pulp mills. Inother words, the invention is capable of fitting chlorine dioxidebleaching of pulp into a bleaching plant in an overall economical andenvironmentally acceptable manner.

[0008] The method of the invention is characterized in that the chlorinedioxide stage comprises successively at least a first chlorine dioxidestep, and an acid treatment step, and that the pH of the first chlorinedioxide step is adjusted so that the final pH of the pulp in the firstchlorine dioxide step is over 4. The conditions during the firstchlorine dioxide step are such that the hexenuronic acid groups in thepulp do not react with chlorine dioxide.

[0009] As discussed above, it is known that hexenuronic acid groups areremoved by hot acid treatment prior to chlorine dioxide treatment.According to the invention it has now been discovered that the ClO₂treatment can also be performed in a bleaching sequence before the hotacid treatment if reactions between the hexenuronic acid groups of thepulp and the chlorine dioxide are prevented. Thus, a D stage maycomprise, successively, a D step and an A step. According to the methodof the invention, the chlorine dioxide step of the DA stage is performedso as to have a final pH of over 4, preferably over 5. The highestdesirable pH value at the end of the first ClO₂ step is normally about7. Conventionally, the final pH of the first or the second chlorinedioxide bleaching stage is less than 4, typically 1-3.5. Surprisingly,it has been found out that hexenuronic acids do not react with chlorinedioxide at the pH range of over 4 and thus no chemical is consumed byreactions with chlorine dioxide. Chlorine dioxide is reduced to chloritebut it does not decompose further. The chlorine dioxide dose in thisstep is about 0.1-1.5% active Cl (1-15 kg active Cl/admt), preferablybetween about 0.5-1.0% active Cl (5-10 kg active Cl/admt).

[0010] In the acid treatment (A) step, the conditions are typically asfollows:

[0011] pH 2-5, preferably between 2.5-4;

[0012] temperature over 80° C., preferably between 90-110° C.; and

[0013] time 30-300 minutes, preferably at least t minutes, where

[0014] t=0.5 exp(10517/(T+273)−24)(t=0.5 e^(((10517/(T+273))−24))), inwhich T (° C.) is the temperature of the acid treatment.

[0015] According to the invention, the temperature in the D step of theDA stage is preferably over 70° C., preferably over 75° C., mostpreferably between 80-100° C., which is higher than the temperature of aconventional D stage. Thus, the temperature in the D and the A steps isessentially the same and there is no special need to cool or to heat thepulp between the steps, which is advantageous for energy economy.However, the invention is not limited to high temperature in the D step,but rather the D step may also be performed at the conventional D steptemperature of below 70° C.

[0016] The treatment time in the D step of the invention is short, lessthan 10 minutes, preferably 30 seconds to 3 minutes. Conventionally, thechlorine dioxide treatment time is over 30 minutes, even 120 minutesdepending on the temperature and therefore a conventional D steprequires its own reactor. The DA stage according to the invention may bepracticed by performing the acid treatment in a reactor tower butbecause of the short retention time the D step preceding the A step maytake place, for example, in the feed line of the A step tower. The pulpflowing in the line is heated to the desired temperature, for exampleabout 90° C., and chemicals, such as chlorine dioxide, and alkali oracid if necessary to adjust the pH, are mixed into the pulp. The pulpflows under these conditions for the required period, e.g. one minute,and subsequently, acid is added to the pulp and it is supplied into theacid tower.

[0017] By providing chlorine dioxide treatment before acid treatmentaccording to the invention, the acid demand in the A step is reduced asthe reactions of chlorine dioxide with the pulp lignin producehydrochloric acid and organic acids as byproducts.

[0018] It is characteristic of a preferred embodiment of the inventionthat the chlorine dioxide stage as described above comprises in additionto the D and A steps described above a second D step, i.e. the chlorinedioxide stage comprises a first chlorine dioxide step, then a hot acidtreatment step, and then a second chlorine dioxide step (DAD, or D/A/D).The second ClO₂ step is performed in acidic conditions, in other wordsthe pH may be less that 2 but preferably is between 2-4. Thus, nospecial adjustment of the pH is needed after the A step. Adjustment ofthe temperature is not necessary, either, but rather the entire chlorinedioxide stage may be practiced essentially at the same temperature asthe A step, and no special adjustments of the temperature are neededbetween the steps.

[0019] The treatment time in the second D step is preferably also short,less than 10 minutes, preferably 1-5 minutes, and it may be performedfor example in the discharge line of the A step tower prior to thewasher following the DAD stage. Chlorine dioxide is added to the pulpflowing in the discharge line and the temperature and the pH areadjusted if necessary. The required chlorine dioxide dose is typically0.5-2.0% active Cl (5-20 kg active Cl/admt). The acid treatment removeshexenuronic acids from the pulp and therefore the amount of chlorinedioxide chemical required in the second ClO₂ step is smaller than withpulp from which hexenuronic acids have not been removed.

[0020] In the (DAD) embodiment of the invention, the chlorine dioxidetreatment may be considered to have been divided into two relativelyshort partial treatments. Both steps may be practiced at a hightemperature; therefore the temperature and the treatment time must bechosen so as to cause as little viscosity loss in the pulp as possible.Two partial treatments contribute to a more homogenous brightness of thepulp than a single longer treatment.

[0021] The DA stage may be also practiced by chelating the pulp withEDTA, DTPA or a corresponding compound, after the A step in order toremove harmful metals; i.e. a DAQ stage may be utilized. Providing achelating treatment in connection with the DA stage is advantageousparticularly if the bleaching sequence comprises a later bleaching stagewith a chemical such as peroxide which is adversely affected by heavymetals. The Q step may be performed at essentially the same temperatureas the preceding D and A steps. The pH may be in the range of 3-6.

[0022] The total amount of chlorine dioxide needed in the DA stage isnot greater than the amount needed in an AD stage although the first Dstep is performed before the acid treatment. The final pH of over 4 inthe first D step prevents reactions between chlorine dioxide andhexenuronic acids. A DA stage has produced pulp of at least as goodquality as an AD stage. An advantage provided by a DA stage, or a DADstage, is that, compared with an AD stage, a DA or a DAD stage may beeffected with simpler apparatus as each step does not require its owntower. However, the invention may also be practiced so that one or bothof the D steps are performed in towers of their own.

[0023] A disadvantage of the AD stage may be the release of malodorousgases in the acid step if the pulp after washing still contains adequateamounts of sulfur compounds from digestion. When the D step precedes theA step, malodorous gases are oxidized by the bleaching chemical duringthe DA stage.

[0024] The method of the invention is applicable to chemical cellulosepulps manufactured by the sulfate process and other alkaline methods,which pulps typically contain hexenuronic acid compounds. The pulp to betreated has preferably been oxygen delignified after digestion andbefore the chlorine dioxide stage. The DA stage according to theinvention may be at the beginning of the bleaching sequence, for exampleafter oxygen delignification, or later in the sequence, for exampleafter a peroxide stage. Therefore the invention is advantageouslyapplied for example in connection with the following treatment/bleachingsequences or partial treatment/bleaching sequences:

[0025] digestion-O-DAD-E;

[0026] digestion-O-DAD-E-DN-D; or

[0027] digestion-O-Q-OP-D/A/Q-PO,

[0028] in which:

[0029] “-” indicates a wash between the stages,

[0030] “O” indicates a delignification stage utilizing at least oxygen,

[0031] “Q” indicates removal of metals by chelating,

[0032] “A” indicates a hot acid treatment as described above,

[0033] “E” indicates an alkaline bleaching stage, and

[0034] “OP” or “PO” indicate a bleaching stage based on oxygen and/orperoxide, possibly at least part of the steps being pressurized and Oindicating oxygen chemical and P peroxide, and the first letter in eachstep signifying the main effective bleaching chemical and the secondletter, when used, designating a bleaching chemical supporting thebleaching reaction.

[0035] According to one aspect of the present invention a method oftreating chemical cellulose pulp from an alkaline pulping process (e.g.the kraft process) in a chlorine dioxide stage is provided. The methodcomprises: (a) In the chlorine dioxide stage, bleaching the chemicalcellulose pulp in a first chlorine dioxide step, and adjusting the pH ofthe pulp in the first chlorine dioxide step so that the final pH of thestep is over 4. And then, (b) in the chlorine dioxide stage effecting anacid treatment of the chemical cellulose pulp at a pH of between 2-5 andat a temperature of over 80° C. Preferably (a) is practiced so that thefinal pH of the first chlorine dioxide step is over 5, and so thathexenuronic acid groups in the pulp substantially do not react withchlorine dioxide. In the method (a) is also preferably further practicedso that the temperature in the first chlorine dioxide stage is over 70°C., preferably over 75° C., and most preferably between about 80-100° C.Also (a) is further practiced so that the treatment time in the firstchlorine dioxide step is less than ten minutes, preferably between 30seconds-3 minutes. The chlorine dioxide dosage during the practice of(a) is preferably between about 0.5-1.5% active chlorine.

[0036] Also preferably (b) is practiced so that the pH is between 2.5-4,the temperature is between 90-110° C., and the time is between 30-300minutes, preferably pursuant to the formula described above. Since (a)and (b) are in the same stage there is no intermediate (i.e. betweenstep) washing.

[0037] Following (b) the method preferably further comprises (c)bleaching the chemical cellulose pulp, after (b), in a second chlorinedioxide step. Preferably (a)-(c) are practiced so that the treatmenttemperatures in the first chlorine dioxide step, the acid treatmentstep, and the second chlorine dioxide step, are substantially the same(between about 90-100° C., e.g. about 95° C.). During the secondchlorine dioxide step the treatment dosage is preferably between about0.5-2.0%, and the temperature and time parameters are substantially thesame as for the first chlorine dioxide step.

[0038] The method may also further comprise (d) treating the chemicalcellulose pulp with chelating agent (such as EDTA, DTPA, mixturesthereof, or other chelating agents) after (a) and (b), especially iffurther treatment of the pulp is to be provided in a peroxide or otherbleaching stage sensitive to the presence of metals. Also all of steps(a) through (c) may be practiced in a line leading up to an acid tower,in the acid tower, and in a line leading from the acid tower to awasher. All of the steps DAD of the stage are of course practicedwithout between step washing, inherent in the fact that they are part ofthe same stage.

[0039] It is the primary object of the present invention to provide forthe advantageous treatment of chemical cellulose pulp from alkalinepulping processes with chlorine dioxide so as to minimize the use ofchlorine dioxide, minimize the amount and expense of equipmentnecessary, enhance the energy economy of the process, and minimize therelease of malodorous gases, such as sulfur compounds. This and otherobjects of the invention will become clear from an inspection of thedetailed description of the invention, and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 schematically illustrates by way of example a preferredapparatus for carrying out the method of a preferred embodiment of theinvention, that is treating cellulose chemical pulp from an alkalineprocess in a DAD stage.

DETAILED DESCRIPTION OF THE DRAWINGS

[0041] In the exemplary embodiment of FIG. 1, chemical cellulose pulp,e.g. produced by kraft pulping, is transferred from a precedingtreatment stage 10 by a high consistency pump 12 to an acid tower 18.The consistency of the pumped pulp is between about 6-25%, preferablybetween about 8-18%. The preceding stage 10 may be a wash stagefollowing digestion, or more often an oxygen delignification stagefollowing digestion, or a wash following oxygen delignification.

[0042] Alkali or acid is added to the pulp in order to adjust theinitial pH for the chlorine dioxide treatment or the first D step sothat the final pH is over 4, preferably over 5, in order to preventreactions between chlorine dioxide and hexenuronic acids. The alkali andthe chlorine dioxide may be added directly to the pump 12 asschematically illustrated in FIG. 1, or may be injected into duct 15between the pump 12 and the tower 18, or added to a conventional mixer14 provided for this purpose. The typical chlorine dioxide dosage added(e.g. at 12) is between about 0.5-1.5% active Cl.

[0043] Steam is preferably added to the pulp in order to raise thetemperature typically up to 80-100° C. Steam may be added prior to thepump 12 in a conventional steam mixer (not illustrated) or mixed in theduct 15 following the pump 12. Alternatively, the pulp may be heatedindirectly in the duct 15 by a conventional heat exchanger (not shown).

[0044] Thus, the first D step is performed in the feed line 15 of theacid tower 18 while the pulp flows in the line 15. The treatment time inthis step is typically between 30 seconds to 3 minutes.

[0045] The first D step is followed by an acid treatment in the tower18. After the chlorine dioxide treatment the pH of the pulp is adjusted(e.g. decreased), if necessary, to the level 2-5 required by the acidtreatment in 18 by adding acid (sulfur acid, hydrochloric acid, wasteacid from the production of chlorine dioxide, or any suitable organicacid) to the pulp in the mixer 14. Also the temperature may be adjustedif necessary but according to the invention all the steps of the DADstage are preferably practiced at about the same temperature, forexample about 95° C., so that no special temperature adjustment isrequired between the steps.

[0046] The feed and flow of pulp to the acid tower 18 should be as evenas possible by using a conventional distributor 16 or a doctor. Thedistributor 16 is preferably as disclosed, for example, in U.S. Pat. No.4,964,950, and its use for the above purpose is described in Finnishpatent publication no. 94442. If the device 16 has adequate mixingproperties and if desired, acid may be added to the device 16 and thuseven avoid the use/purchase of the mixer 14.

[0047] Pulp flows from the device 16 to the reactor tank 18 dimensionedfor the treatment time required by the acid treatment step A, forexample 120 minutes at 95° C. Using a distributing feeder 16 asdescribed in FI publication no. 94442, all portions of the tower 18 arefilled evenly and the pulp column rises up evenly in the tower 18 sothat harmful channeling cannot occur. Correspondingly, the top of thetower 18 is provided with a conventional discharger 20 or a dischargedoctor to guide pulp to the discharge line 24 of the tower.

[0048] The second D step is performed in the discharge line 24 fromtower 18 in order to bleach the pulp. Chlorine dioxide may be added tothe pulp via the acid tower discharger 20 or to a pump 22 in thedischarge line 24. Also a separate mixer (not illustrated) may beprovided in the line 24 for addition of chemical, preferably between0.5-2.0% active Cl.

[0049] The addition of chemicals to the pulp is effected so that aproper treatment time for carrying out the second D step in thedischarge line 24 is provided before the conventional washer 26 isreached. The retention time is less than 10 minutes, preferably betweenabout 1-5 minutes. The treatment temperature of the second D step ispreferably the same as that of the first D and A steps; thus no specialadjustment of the temperature after the A step is needed here either. Ifdesired, pulp may be heated or cooled in a conventional heat exchanger(not shown) or by direct feeding of steam. A higher temperature (e.g.between about 75-110° C.) than conventionally is, however, required inorder to obtain adequate bleaching during the short treatment time inline 24.

[0050] The pH of the second D step is lower than that of the first Dstep. Typically the pH of the second D step in line 24 is between about2-4; thus adjustment of the pH is usually not needed after the A step.If necessary, the chemical adjusting the pH may be added at the sametime and place as the chlorine dioxide (e.g. in discharger 20).

[0051] Pulp was bleached in a DAD stage in the manner described aboveand subsequently the pulp was washed in the washer 26 and transported tofurther treatment. Typically the further treatment (subsequent stage) isan E stage. The results of one example of this treatment are:

EXAMPLE

[0052] Oxygen-bleached initial pulp was tested in a laboratory, thekappa number of which was 11.9, viscosity 1061 ml/g and the ISObrightness 49.2%. The pulp was treated with the following sequences:

[0053] 1. DAD-E_(o)-D_(N)-D

[0054] D:

[0055] ClO₂ dose 1.0% as active Cl

[0056] time 1 minute

[0057] final pH 5.3

[0058] temperature 95° C.

[0059] A:

[0060] time 180 minutes

[0061] pH 3.5

[0062] temperature 95° C.

[0063] D:

[0064] ClO₂ dose 1.25% as active Cl

[0065] time 2 minutes

[0066] final pH 2.5

[0067] temperature 95° C.

[0068] E_(o)

[0069] 85° C., 60 minutes, 1.25% NaOH, O₂ 4 bar, final pH 11.7

[0070] kappa number after treatment 2.3

[0071] viscosity after treatment 890 mg/l

[0072] brightness after treatment 70.2% ISO

[0073] D_(N)

[0074] 75° C., 180 minutes, 1.8% ClO₂ as active Cl, 0.3% NaOH, final pH3.3

[0075] N (neutralization) pH 8.3

[0076] D

[0077] 75° C., 180 minutes, 0.8% ClO₂ as active Cl, 0.1% NaOH, final pH5.1

[0078] The final bleached pulp had a viscosity of 868 ml/g, and abrightness of 89.9% ISO. In the second test the same initial pulp wastreated

[0079] 2. AD-E_(o)-D_(N)-D

[0080] A:

[0081] time 180 minutes

[0082] pH 3.5

[0083] temperature 95° C.

[0084] D:

[0085] ClO₂ dose 2.25% as active Cl

[0086] time 1 minute

[0087] final pH 2.1

[0088] temperature 95° C.

[0089] E_(o)

[0090] 85° C., 60 minutes, 1.25% NaOH, O₂ 4 bar, final pH 11.6

[0091] kappa number after treatment 2.7

[0092] viscosity after treatment 890 mg/l

[0093] brightness after treatment 69.4% ISO

[0094] D_(N)

[0095] 75° C., 180 minutes, 1.8% ClO₂ as active Cl, 0.3% NaOH, final pH3.4

[0096] N (neutralization) pH 8.3

[0097] D

[0098] 75° C., 180 minutes, 0.8% ClO₂ as active Cl, 0.1% NaOH, final pH5.1

[0099] The final bleached pulp had a viscosity of

[0100]866 ml/g, and a brightness of 89.9% ISO.

[0101] Based on the above example it may be concluded that a DA stageand an AD stage according to the invention produce pulp of the samequality. Some practical advantages described above may be obtained byeffecting the chlorine dioxide and the acid treatment according to theinvention as compared with an AD sequence.

[0102] In all of the above descriptions all narrower ranges within abroad range are also specifically provided. For example the temperaturerange of 80-100° C. includes 81-95° C., 94-99° C., and all othernarrower ranges within the broad range.

[0103] While the invention has been herein shown and described in whatis presently conceived to be the most practical and preferredembodiment, it will be apparent to those of ordinary skill in the artthat many modifications may be made thereof within the scope of theinvention, which scope is to be accorded the broadest interpretation ofthe appended claims so as to encompass all equivalent methods.

What is claimed is:
 1. A method of treating chemical cellulose pulp froman alkaline pulping process in a chlorine dioxide stage comprising: (a)in the chlorine dioxide stage, bleaching the chemical cellulose pulp ina first chlorine dioxide step, and adjusting the pH of the pulp in thefirst chlorine dioxide step so that the final pH of the step is over 4;and then (b) in the chlorine dioxide stage effecting an acid treatmentof the chemical cellulose pulp at a pH of between 2-5 and at atemperature of over 80° C.
 2. A method as recited in claim 1 wherein (a)is practiced so that the final pH of the first chlorine dioxide step isover 5, and so that hexenuronic acid groups in the pulp substantially donot react with chlorine dioxide.
 3. A method as recited in claim 2wherein (a) is further practiced so that the temperature in the firstchlorine dioxide step is over 75° C.
 4. A method as recited in claim 2wherein (a) is further practiced so that the temperature in the firstchlorine dioxide step is between about 80-100° C.
 5. A method as recitedin claim 4 wherein (a) is further practiced so that the treatment timein the first chlorine dioxide step is less than 10 minutes.
 6. A methodas recited in claim 4 wherein (a) is further practiced so that thetreatment time in the first chlorine dioxide step is between 30seconds-3 minutes.
 7. A method as recited in claim 5 further comprising(c) bleaching the chemical cellulose pulp, after (b), in a secondchlorine dioxide step.
 8. A method as recited in claim 7 wherein (a)-(c)are practiced so that the treatment temperatures in the first chlorinedioxide step, the acid treatment step, and the second chlorine dioxidestep, substantially the same.
 9. A method as recited in claim 1 furthercomprising (c) treating the chemical cellulose pulp with chelating agentafter (a) and (b).
 10. A method as recited in claim 1 wherein (a) isfurther practiced so that the temperature in the first chlorine dioxidestep is over 75° C.
 11. A method as recited in claim 1 wherein (a) isfurther practiced so that the temperature in the first chlorine dioxidestep is between about 80-100°C.
 12. A method as recited in claim 1wherein (a) is further practiced so that the treatment time in the firstchlorine dioxide step is less than 10 minutes.
 13. A method as recitedin claim 1 wherein (a) is further practiced so that the treatment timein the first chlorine dioxide step is between 30 seconds-3 minutes. 14.A method as recited in claim 1 further comprising (c) bleaching thechemical cellulose pulp, after (b), in a second chlorine dioxide step.15. A method as recited in claim 14 wherein (a)-(c) are practiced sothat the treatment temperatures in the first chlorine dioxide step, theacid treatment step, and the second chlorine dioxide step, substantiallythe same.
 16. A method as recited in claim 4 wherein (a) is furtherpracticed so as to provide a chlorine dioxide dosage of between about0.5-1.5% active chlorine during the first chlorine dioxide step.
 17. Amethod as recited in claim 7 wherein (a) is further practiced so as toprovide a chlorine dioxide dosage of between about 0.5-1.5% activechlorine during the first chlorine dioxide step; and wherein (c) ispracticed so as to provide a chlorine dioxide dosage of between about0.5-2.0% active chlorine during the practice of the second chlorinedioxide step.
 18. A method as recited in claim 17 wherein step (b) ispracticed at a pH between 2.5-4, a temperature between 90-110° C., and atime between 30-300 minutes.
 19. A method as recited in claim 18 wherein(a)-(c) are practiced so that the treatment temperatures in the firstchlorine dioxide step, the acid treatment step, and the second chlorinedioxide step, are substantially the same, and between about 90-100° C.20. A method as recited in claim 19 wherein (a) through (c) arepracticed utilizing an acid tower, an inlet line to the acid tower, andan outlet line from the acid tower to a further treatment device; andwherein (a) is practiced substantially completely within the inlet lineto the acid tower, (b) is practiced substantially completely within theacid tower, and (c) is practiced substantially completely in thedischarge line from the acid tower.